No where. The word "vacuum" is an engineering term that refers to a condition of relatively low density, but not pure emptiness. Even the most remote regions of inter-galactic space have densities of about one particle per cubic meter, and there are always some electromagnetic fields present (required for light waves). Most of space consists of plasma - the most common state of visible matter in the universe. So-called "dark matter" may add to this (hypothesized but not directly confirmed).

There are only four dominant naturally-occurring states of matter; earthlings are most familiar with solid, liquid, and gas. However, we are mostly surrounded by plasmas!

On our planet the most prevalent man-made plasmas are the plasmas in lamps. There are primarily two types of plasma-based light sources, fluorescent lamps and high-intensity arc lamps. Fluorescent lamps find widespread use in homes, industry and commercial settings. Find out more at http://www.plasmas.org/rot-home.htm.

Between the Sun and Earth, the space environment is full of particles, and electric and magnetic fields. Our Sun constantly emits plasma, a superheated state of matter, which moves out in all directions at very high speeds to fill the entire solar system and beyond. This unusual state of matter affects spacecraft systems, conducts electricity, and in turn can interfere with radio, television, and telephone signals, upset the navigation systems of ships and airplanes, and cause blackouts. In some instances, it can be dangerous to astronauts out in space and especially on space walks. However, on a more positive note, solar wind also causes the Aurora Borealis also known as the northern lights.

To understand the changing Sun and its effects on the Solar System, life, and society is the goal of the Sun-Earth Connection theme at NASA (see http://sec.gsfc.nasa.gov/). Educators may find useful activities and programs at http://sunearth.gsfc.nasa.gov/. By studying processes that occur in the earth's magnetosphere (where earth's magnetic field has a greater influence than the Sun's interplanetary field), in interplanetary space, and around other planets, we are better able to appreciate the important role of plasmas throughout our plasma universe. This space plasma laboratory is truly our window to the stars.

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Dr. Timothy E. Eastman is Program Manager for Raytheon's Space Science Group working in support of the Space Science Data Operations Office at NASA/GSFC.

Dr. Eastman has more than 25 years experience in the areas of space physics research, and plasma science and its technology applications. He created and maintains the
plasma site "plasmas.org" covering all aspects of plasma science and technology worldwide.

E-mail: timothy.eastman@gsfc.nasa.gov
Web master for the plasma page: www.plasmas.org